0004 — Detect and refuse ADCRYPT; no decryption in v1

Status: Accepted

Context

AD1 has an encrypted variant, ADCRYPT (AES-128/192/256, PBKDF2 with SHA-256/512 per the reference header). A forensic reader that silently produced garbage for an encrypted image would fabricate evidence — the worst failure mode for a forensic tool.

Decision

Detect ADCRYPT from the leading signature and refuse: Ad1Reader::open returns Ad1Error::Unsupported (with the signature bytes) and the auditor emits an AD1-ENCRYPTED finding. Decryption is explicitly out of scope for v1.

Consequences

• No fabricated output: an encrypted image is reported as such, never decoded to plausible-but-wrong bytes.
• Decryption (key derivation, AES) is a later epic; when built it must reuse audited RustCrypto crates and validate against a real encrypted image, never a self-made round-trip.

Alternatives considered

• Attempt decryption now — rejected: scope creep; requires key material and careful crypto, and would delay the core reader.
• Best-effort parse of the ciphertext — rejected: guarantees garbage output; a fail-loud refusal is the only forensically safe behavior.